Consumer electronic manufacturers increasingly view three-dimensional (3D) shaped glass or glass-ceramic components as differentiating design elements that will enable them to continue to evolve their device designs and generate ongoing interest from the marketplace. To service this market, glass manufacturers and component suppliers have developed several competing processes to form 3D shapes from flat glass preforms. These solutions include double-mold processes, such as pressing, where molds contact both sides of the preform, and one-mold processes, such as vacuum forming and pressure forming, where a mold contacts only one side of the preform.
A high throughput precision 3D glass forming process (double-mold pressing, single-mold vacuum forming, or single-mold pressure forming) having a short Takt time can enable lower cost manufacturing. “Takt time” is average unit production time needed to meet customer demand. One of the challenges with reducing Takt time is maintaining the required precision of 2D preform placement. Short Takt time can be more readily achieved in single-mold processes. In double-mold processes, there is high friction between the upper and lower molds due to a need to maintain tight tolerances while pressing the glass. Because of this high friction it is not possible to apply fast pressing action.
Precise preform placement on the mold is required in order to meet tight specifications on the outer periphery of 3D parts and specifically to reduce variability and achieve accuracy in length, width, and height dimensions of the 3D parts. If the preform is placed on the mold inaccurately, the 3D part may still have good compliance to the CAD model in contour dimensions but because of its length, width or height may be out of tolerance and the 3D part may not fit into the final device. Typical tolerances for length, width, and height dimensions range from +/−0.050 mm to +/−0.150 mm. For a 3D part to meet these tolerances, the preform must be placed on the mold with even tighter tolerance, typically within +/−0.050 mm or better, depending on exact specification requirements and the flat preform tolerances.
Traditional part locating/sensing techniques are not capable of meeting all the technical requirements of this application. Machine vision systems, for example, are not tolerant of the high ambient temperatures present above the mold in the forming process. When placed in a location with ambient temperatures that satisfy camera temperature specifications, existing vision systems generally do not have the resolution required to satisfy the micron-level positioning requirements for this application. Vision systems also require very specific lighting that is very difficult to achieve in the preform's glass edge and are further complicated by the changing mold finish due to the surface oxidation that occurs in mold materials at process temperatures.